What is this information used for?

Engineering Geology and the Law

There is a bigger issue in engineering geology than any other aspect of geo with liability. Even though oil and gas decisions often involve a much larger budget/cost rarely do decision have a life or death element. Construction usually does. As almost all construction relies on building on a site the site investigation is fundamental.

In Britain over 30% of all major civil engineering projects are delayed by poor ground conditions. What does poor mean? The fact that the initial site investigation did not properly classify the ground. This leads to over 50% of projects going over budget!!

P-wave Velocity and Rippability

Borehole Spacing dependant on Project Type and ground conditions

Minimum Spacing 10m 30m Maximum Spacing

Boreholes

Project Type

Buildings

Road Sections

30m

300m

Road sections in Hazard area

1m

5m

Typically boreholes should penetrate a depth to 1.5 times the building foundation width Plus if 'sound' bedrock (rockhead) is not encountered in this depth then at least one borehole should penetrate to rockhead

Rotary Coring Gravity-coring Vibro-coring

Hard Rock Testing

Rarely is it necessary to test intact rock strength for bearing capacity as the strength envelopes are well know. (Example table Handout 4, Table 4-1) Also note some primary rock features that can influence strength such as:

Use of Rock Strength/Engineering Properties

Aggregate Crushing Value (AGV) handout 6 Measure the percentage of fines (<2.36mm diameter) left after application of 400kN load for 10 min. Aggregate Impact Value (AGI) Measure the fines after dropping a hammer onto a sample of aggregate Aggregate Abrasion Value (AAV) Measure of mechanical abrasion through comparison of the weight of an aggregate sample before and after it has been abraded (Fig 4-8)

Soil Testing

Routine part of all investigations includes both insitu, field testing and laboratory analysis on samples taken from boreholes, test pits and the surface. Engineering soil is defined as any unlithified (unconsolidated) material (sediment) Description follows Section 8 of BS 5930 Aim - Measure Strength - Determine sensitivity to Failure Initial classification of material based on: ? Grain size ? Mineralogy ? Grain arrangement ? Water content

Atterberg Limits

Measure of consistency of soil - this is the moisture content at which soil behaves in a plastic or liquid fashion As water content increases soil goes from solid to plastic to liquid Plastic Limit (PL) - minimum water content at which soil is rolled into a 3mm diameter cylinder (approximate shear strength of 100kPa) Liquid Limit (LL) - minimum moisture content at which soil will flow under own weight Plasticity Index (PI) - difference between PL and LL, indicates the amount of moisture required to go from liquid to plastic Liquidity Index (LI) - mobility of the soil at a particular moisture content (W) LI=(W-PL)/PI The higher the liquidity index the more unstable a soil is.

Mohr Circles and Soil Shear Strength/Failure

Shear strength expressed by Coulomb Failure Envelope

? ? c ? (? ? P ) tan ?

Changes/differences in Shear Strength (?) can result from Changes in normal stress (? ) porewater pressure (P) changes due to drainage (porosity) differences in the angle of internal friction (? ) due to interparticle roughness and cohesion (c) weathering reducing cohesion and angle of internal friction remoulding of the sediment

Measurement of Shear Strength

Measurement of Shear Strength

Standard Penetration Test - SPT (typically field test) Test 19 of BS 1377:1975 A 51mm split tube is driven into sediment for 150mm. 64kg weight hammer dropped onto it over a distance of 760mm Number of blows recorded to drive tube a further 300mm

Reporting - Site Reports and Engineering Geology Maps

- Synthesis of observations and measurements (handout 8) - Based on the aims of the site investigation and its scale - Engineering Geology Maps - Landsystem Map - thematic map used to classify larger areas of land based on broad engineering classification, somewhat follows OS drift maps - Detailed Site Plan - geomorph, geology, topo, point engineering data, groundwater, hazards

Summary

What is critical to Engineering Geology?

- Know your rocks and soils - Know their properties as a whole and as constituent parts - Know the conditions/properties within the rocks and soils - moisture content etc. - Know what processes they have undergone and are undergoing - e.g. weathering, diagenesis - Know how they are used - loaded, stressed etc

Hydrogeology

hydrology - the study of water hydrogeology - inter-relationship of geologic materials and processes with water (c.f. geohydrology)

The Hydrologic System and the Hydrologic Cycle (fig) inflow = outflow +/- changes in storage

mixed grain sizes reduce porosity

Fabric (rock type) is important

Factors influencing Porosity - cements & fracturing

Cementation e.g. calcite, dolomite, silica

Diagenesis (rock type) is important

Hydrogeological factors of geophysical interest

Specific yield - ratio of the volume of water that drains from a saturated rock owing to attraction of gravity, to the total rock volume (Sy) Specific retention - ration of water retention to total rock volume (Sr)specific retention specific yield

Biological Pollutants and Ground Water

Smaller the grain size smaller the pore size smaller the microorganisms that can be filtered

Major cause of contamination is biological contamination e.g. bacterial diseases such as bubonic plague in 17thcentury

Hydrogeology - Groundwater flow

hydraulic gradient - with all other factors constant the rate of ground water movement is the hydraulic gradient or change in head per unit of distance in a given direction. potentiometric surface - surface to which water will rise in well cased to an aquifer. potentiometric map - contour map of potentiometric surface of a particular hydrogeologic unit. Usually measured using a piezometer water table is the potentiometric head (surface) for an unconfined aquifer, here pore water pressure is equal to atmospheric pressure.